Lewis & Short

1. aethra, ae, f., = αἴθρα [v. aestas],

  1. I. the upper, pure air, the bright, clear, serene sky: aetheris splendor, qui sereno caelo conspicitur, Serv. ad Verg. A. 3, 585 (poet.): flammea, Jul. ap. Macr. 6, 4 (Trag. Rel. p. 228 Rib.): siderea, Verg. A. 3, 585.
  2. II. Transf., like aether, the sky, air, heavens: surgere in aethram, Lucr. 6, 467: volans rubrā ales in aethrā, Verg. A. 12, 247; so Sil. 4, 103; Stat. S. 1, 2, 135 al. (but in Cic. N. D. 2, 15, 42, the correct read. is aethere, B. and K.).